Stent assembly and device for application thereof

ABSTRACT

A stent assembly ( 2 ) includes a stent capable of assuming a contracted and an expanded configuration. A membrane of a biocompatible material impermeable to molecular transport across the membrane is wrapped around the stent more than one full turn. There is also disclosed a device for performing anastomosis, including a stent assembly ( 2 ) in a contracted state and a holding and gripping device ( 12 ). The handle has a handle portion ( 14 ) and a support structure ( 16, 18 ) for accommodating the stent assembly. There is a locking loop ( 20 ) that can be tightened around the stent assembly when it is positioned on the support structure, and released to allow the stent assembly to expand to a nominal size.

The present invention relates generally to devices and methods forconnecting severed vessels, e.g. arteries and veins, and in particularto a covered stent and a method and system for applying the coveredstent.

BACKGROUND OF THE INVENTION

Currently employed techniques for connecting blood vessels in thecontext of transplantation surgery and other similar kinds of surgicalprocedures are based on suturing the ends of the vessel with acontinuous suture. This is a time consuming procedure that often mayrequire using a microscope and is rendered complicated by limitedaccessibility to the site of surgery in the body. There are suggestionsin the patent literature for facilitating and simplifying thisprocedure, but they nevertheless suffer from lack of ability to meetsome essential requirements. Namely, the method should

be easy to perform in a short time

provide immediate and efficient sealing under physiological pressure andtemperature conditions

maintain patency (i.e. should not induce trombosis or inflammation)

be user friendly

provide devices suitable for industrial production

The patent literature comprises a vast number of patents relating tostents and devices for manipulating stents.

One example of a prior art stent comprising shape memory is disclosedi.a. in U.S. Pat. No. 5,354,308 (Simon et al). It relates to a stentcomprising a wire skeletal frame, said frame being adapted to assume afirst condition in which said frame is expanded, and a second contractedcondition. The wire frame comprises a metallic compound of nickel andtitanium, said compound in said second condition retaining memory ofsaid first condition. When heated to a selected body temperature, itassumes said first condition in which said frame is greatly expandedrelative to said second condition.

SUMMARY OF THE INVENTION

The object of the present invention is to provide an improved devicethat meets the desired requirements outlined above, in particular toprovide immediate and efficient sealing under physiological pressure andtemperature conditions in combination with providing long lastingpatency. This object is achieved in a first aspect with a device asdefined in claim 1.

Thereby a stent, capable of assuming a contracted and an expandedconfiguration, and preferably made of a shape memory material, isprovided in a reduced size and a piece or membrane of foil material,e.g. polymer, is tightly wrapped around the stent as a separate entity.Thus, the ends of said membrane may either be loose, i.e. they are notattached or connected to the stent in any way, or the inner end part ofthe membrane may be fixed to the stent at a small fraction of thecircumference of the stent. This means that if the stent is allowed toexpand to its nominal size, the membrane will follow in this expansion,and accommodate to the varying diameter. Preferably, the length of themembrane is selected such that in a fully expanded state of the stent,the ends of the polymer membrane or foil will overlap only by a fractionof the circumference, i.e. the membrane is wrapped around the stent atleast slightly more than one full turn.

In order to provide and ascertain complete blood compatibility, thesurfaces of those parts that come into contact with body fluids insidethe vessel in which they are located, will preferably be treatedchemically or biologically. The preferred treatment is to coat thesurfaces with heparin, although other surface treatments are alsopossible and within the inventive concept.

Also, it should allow a stent to be applied or introduced by a simpleand quick method. Thus, in a further aspect there is-provided a devicefor enabling easy application of the stent and for connecting bloodvessels. This device is defined in claim 12.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description given hereinbelow and the accompanying drawingswhich are given by way of illustration only and thus not limitative ofthe present invention, and wherein

FIG. 1 is an illustration of a stent in general (not according to theinvention);

FIG. 2 a illustrates an embodiment of the device according to theinvention wherein a stent is provided in a contracted state;

FIG. 2 b illustrates the same device as in FIG. 2 a but wherein thestent has been released and expanded to a nominal size;

FIG. 2 b schematically illustrates a variation of the stent and membraneassembly according to the invention wherein the membrane is attached tothe stent;

FIG. 3 is a perspective view of a device for anastomosis comprising astent/membrane assembly as shown in FIG. 2;

FIG. 4 illustrates an embodiment of the membrane of the assembly;

FIG. 5 a shows an embodiment of end stops according to the invention;

FIG. 5 b shows an alternative embodiment of end stops;

FIG. 5 c shows a cross-section through a blood vessel with a stent as inFIG. 5 a;

FIG. 6 shows an embodiment where the membrane is secured with a gluedot; and

FIG. 7 a-b are cross-sections through a stent and balloon catheterassembly in non-inflated and inflated conditions, respectively.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Stents for anastomosis are supplied in numerous designs, and FIG. 1illustrates one type of stent suitable for use in the present invention.It comprises a tubular member made from expandable shape memory metal.An example of a suitable material is NiTinol™, which has the capabilityto regain a given shape when exposed to a given temperature. For thepurposes of the present invention, it should be capable of assuming adesired shape when exposed to body temperature, e.g. about 37° C.

Stents of this type are commercially available, e.g. under the tradename SelfX from Jomed A B, Helsingborg, Sweden. Thus, the actualspecific design of the stent member as such does not form part of thepresent invention.

FIG. 2 a-c illustrate embodiments of the basic idea behind theinvention.

A problem often encountered in anastomosis is that the finished joint isvery difficult to make absolutely leak-proof under the physiologicalconditions of pressure prevailing in blood vessels. Therefore, accordingto the invention a stent has been provided with means for completelysealing the joint or seam between the two ends of the blood vessels onwhich the anastomosis has been carried out. Thus, a stent assembly 2according to the present invention comprises a stent member 4, asdescribed above and capable of assuming a contracted and an expandedconfiguration, and a piece or sheet/membrane 6 made of a material thatis biocompatible and impermeable to liquids and molecular transport,tightly wrapped around the stent member when the stent is in acontracted state. Thus, at least one end 10 of said membrane in thisembodiment is loose, i.e. it is not attached or connected to the stentin any way. This means that if the stent is allowed to expand to itsnominal size (see FIG. 2 b), the membrane will follow in this expansion.Preferably, the length of the membrane is selected such that in a fullyexpanded state of the stent, the ends of the polymer membrane or foilwill overlap only by a fraction of the circumference. This implies thatthe membrane will be wrapped a few turns around the stent in itscontracted state. Thereby the membrane will exert two functions: i) byvirtue of the total thickness of the coiled membrane, it will offerenough resistance to prevent premature expansion of the stent: and ii)after expansion of the stent inside a blood vessel, the membrane will beprovided essentially as a mono-layer, ends overlapping by only a few mm,in a standard size of the stent by say 5-10 mm. The degree ofoverlapping is not critical, and will of course also depend on the sizeof the stent.

An important advantage connected with using a non-porous membrane isthat it efficiently separates the fresh anastomosis joint from thecirculating blood. This joint would otherwise inevitably lead toactivation which can trigger a number of processes causing problems,such as thrombosis, cell proliferation or hyperplacia.

In another embodiment the inner end 8 of the membrane contacting thesurface of the stent can be attached 9 to said surface, see FIG. 2 c, bygluing, welding or other suitable method.

The material for the membrane is suitably a polymer material, but othermaterials are possible as long as they are biocompatible and meet therequirement of impermeability. Preferably the material can be alaminated material wherein the two layers-have slightly differentproperties in terms of thermal expansion coefficients. This means thatwhen subjecting the membrane to a temperature change, the upper andlower layers will expand or contract at a slightly different rate. Thiswill cause the membrane to become slightly curved. In such a conditionit is easier to wrap the membrane around a stent. Furthermore, if thecurvature of the membrane can be made to be smaller than the curvatureof the stent, at least in the expanded (nominal) condition of the stent,the membrane will accommodate much better to the circumference of thestent, and there will be no tendency for the membrane to spring off oraway from the surface of the stent. It can be advantageous to apply acold spray to the membrane when it has been mounted on the stent, sincethe different thermal coefficients of the layers will cause the membraneto tighten further around the stent.

A suitable laminate film meeting the above requirements is a laminate ofpolyethylene/polyester (e.g. Steriking ESE 1240 from Wipak, Nastola,Finland). Biodegradable materials are also possible and could be ofbenefit in certain circumstances.

The basic idea is that a contracted stent having an impermeable sealingmembrane wrapped around it is provided. The contracted state of thestent is achieved by simply cooling it, e.g. by a cold spray. Thecontracted state is preserved by providing some clamping means aroundthe assembly of stent/membrane. A presently preferred embodiment of adevice for anastomosis 12 according to the invention is shown in FIG. 3.

Thus, the device 2 according to the invention is provided with a handle14 (or manipulator piece), having a support structure 16 for thestent/membrane assembly 2 to rest in. This can be in the form of asemi-circular surface 18 having a radius of curvature corresponding tothe diameter of the assembly 2. The surface 18 of the support structurepreferably has a relatively limited width, i.e. it may resemble a fork,in order not to occupy too much space at the site of the blood vesselanastomosis during application of the stent. The handle 14 is preferablytube shaped or at least it has a central channel for accommodating athread 20 (described further below), and opening in an aperture 22 inthe surface 18 of the support structure 16.

The thread 20 is provided around the stent/membrane assembly 2 onesingle turn, to tighten the assembly and thus preventing it fromunwinding. In order to further reduce the tendency of the end of themembrane to deflect from the circumference of the assembly, therebycausing potential problems during application or insertion of theassembly into a blood vessel, the outer (free) end of the membrane canbe made to exhibit a triangular shape (indicated in FIG. 3), such thatthe tightening thread runs across the apex 21 of said triangle.

The loose ends 24, 26 of the thread 20 are passed through the abovementioned aperture 22 in the support structure 16. The thread ends 24,26 preferably exit at the opposite end 28 of the handle 14, where theyare secured by a suitable clamping means so that it is not possible forthe stent assembly to inadvertently expand before it has beensuccessfully positioned in the site where the anastomsis is to beperformed. The ends can simply be tied to an anchoring element 30 bytying a knot, or alternatively there can be provided a frictionengagement in the form of a slit (not shown), in which the thread endscan be clamped. Of course it is equally possible to let the thread exitanywhere else on the handle, but at present it appears most convenientto simply let the tread 20 pass through the inner lumen of the handle toits rear end 28.

The details of the locking or clamping means do not form part of theinvention per se, and many other options are available, as realized bythe skilled man who can easily find alternatives without inventive work.

In a further embodiment of the invention the thread 20 is attached tothe membrane 6 at its outer end 10, see FIG. 4. The means of attachingcan be of different kinds, but a method which at present is preferred isto design the membrane so as to exhibit a tab 32 located at the apex ofthe triangularly shaped end portion 21. The end of the thread 20 isformed to a loop 34, and then the thread 20 is positioned on said tab 32such that the loop 34 extends out from the tab, as shown in FIG. 4 andsuch that the end 36 of the thread 20 runs parallel with the thread.Then a small piece 38 of membrane material is positioned on top of thetab 32 at the inner end thereof, and another small piece 40 of membranematerial is positioned on top of the tab 32 at the outer end thereof,such that the thread is sandwiched between the tab and the pieces ofmembrane material. Alternatively one single piece of membrane materialhaving about the same size as the tab 32 is placed on the threads toform a sandwich assembly. Heat is then applied by pressing the sandwichstructure between two heated members (e.g. two heated platens, heatedpliers or the like) so as to weld the material 38, 40 pieces onto thetab 32 thereby fixating the thread on the tab. In this embodiment therewill only be one thread end 25. Thus, when the membrane is wrappedaround the stent in a crimped or contracted state, there will be onlyone single strand of thread extending through the handle 14 of themanipulator 12.

Because of the double layer of membrane material on the tab, and thethreads between the layers, the tab 32 will become substantially stifferthan the membrane as a whole. This can be used to advantage for securingthe stent/membrane assembly in place in the blood vessel afteranastomosis.

Namely, it may occur that the stent and the membrane are displacedrelative to each other once they have been inserted in the blood vesseljoint. One reason for this to happen is that the two blood vessels to bejoined in the anastomosis procedure do not always exhibit exactly thesame diameter. When this is the case, i.e. the diameters differ slightlythe tension in the larger vessel will of course become less than in thesmaller vessel which will be more tightly positioned on the stent. Suchdifferences in tension may give rise to a force vector in thelongitudinal direction of the stent, and can cause a dislocation ordisplacement of the stent or the membrane.

Manipulations during surgery will of course mechanically affect thestent/membrane assembly which can cause the mentioned dislocation tooccur.

Thus, in operation of the assembly according to this embodiment, thestent will be placed such that the tab 32 is positioned in the actualjoint, extending out therefrom. When the thread 20 subsequently isreleased, thereby causing the stent to expand, it will be an easy matterto pick up the thread at a point adjacent the attachment on the tab 32by e.g. a pair of forceps and then pull the thread 20 such that theloose end 25 runs around the stent assembly. When the entire thread hasbeen pulled out it can be secured in the tissue e.g. by suturing, andthe excess length of thread is simply cut off. In this way the membraneis secured from being dislocated. In particular, if the inner end 8 ofthe membrane 6 is attached to the stent, the stent will also be securedby this operation and cannot move.

In another embodiment of the invention, the stent and membrane assemblyis usable together with a balloon catheter for dilatation purposes. Inthis embodiment the stent need not be made of a shape memory material,i.e. it need not be self expandable. However, it must of course stillexhibit the ability to assume both a contracted and an expanded state.For this application the stent carrying the membrane is positioned overthe balloon on a catheter. The entire assembly is inserted into anartery to the location where a dilatation is desired. By inflating theballoon, the stent will expand and the membrane will follow theexpansion and cover the stent by slightly more than one turn. FIG. 7 aand 7 b schematically illustrates use with a balloon catheter in anon-inflated state (FIG. 7 a), and an inflated state (FIG. 7 b). Theballoon is designated 70.

In order to be usable on a catheter that is to be inserted into theblood vessel tree structure, as far as into the coronary vessels, itmust be possible to keep the membrane in place on the stent during theinsertion operation. It would not suffice to just wrap the membranearound the stent, since it would easily slide off from the correctposition due to the friction encountered inside the blood vessels.

Therefore, as can be seen in FIG. 5 a, which is a schematic perspectiveview of a generally cylindrical stent 50, there is provided a number ofend stops 52, at least two, preferably three or more (four shown in FIG.5 a), distributed along the periphery of the stent at its ends. Theseend stops can be provided in several ways. In one version the threadmaterial (suitably metal) from which the stent is made, can be used toprovide very small upwardly directed, protruding pins or pegs.Alternatively, there could be provided pellet like structures 54, seeFIG. 5 b. If the stent is made from a moldable material such as apolymer, the pins can be provided in the molding of the stent itself.

The end stops would have an extension in the radial direction that doesnot exceed the thickness of the wrapped membrane, in order not topresent any obstacles during the insertion operation. For example, atypical thickness of a membrane is about 50 μm, and in a contracted statof the stent, the membrane typically is wrapped three turns around thestent. Thus, the total thickness of the membrane would be about 150 μm(In FIG. 7 a, the several turns of the membrane 58 are shown as onelayer for clarity). Typically, the end stops would protrude about 100 μmoutwards, such that when the stent has expanded, and the membrane iswrapped only one turn (plus some overlap), the membrane would still bekept in place longitudinally by the provision of said end stops, as canbe seen in FIG. 7 b.

Furthermore, once the stent is expanded, the end stops could be made toextend above the surface of the membrane. Thereby, the pins or pegs canact as hooks, actually gripping the inner wall 56 of the blood vessel,thereby effectively keeping the stent 50 in place, preventing it frombeing displaced in the longitudinal direction of the vessel, see FIG. 5c and FIG. 7 b. At the same time the end stops continue to act as such,i.e. they will prevent the membrane 58 from being slid off the surfaceof the stent.

In the case of the embodiment described earlier, wherein the stent ismade of a shape memory material (for use as a connector), the stent canbe given a memory of the expanded state in which the pins or pegs have aradial extension, thereby acting as hooks in the same way as justdescribed, but in the contracted state, i.e. when cooled down, the pinswill assume a longitudinally extended position, or even slightly bentinwards. In this way, the stent can be easily inserted into the vesselends without the pins obstructing the passage. When put in place in thevessel the stent will expand. When the vessel ends are brought togetherfor suturing, the pins will have changed their direction (i.e. regainedthe “remembered” configuration) and will grip the inner wall of thevessel, thereby securing the stent in correct position.

For the embodiment usable with balloon dilatation catheters, themembrane 58 must be secured in its wrapped condition on the stent 50.This can be ascertained by attaching a glue dot 60 on the outer end edgeof the membrane, so as to fix the edge against the membrane surface. Inthe case of a triangularly shaped end edge 62, the apex of the trianglewould suitably constitute the point of attachment, see FIG. 6.

The stent/membrane assembly can be made by the following procedure:

The surfaces of the stent and the membrane respectively are providedwith surface bound heparin, by methods known per se by the skilled man.One example is disclosed in our own Swedish patent 9102798-7(corresponding to U.S. Pat. No. 5,529,986).

Other surface treatments are also possible and within the inventiveconcept. It may for example be desirable to provide various kinds ofmedicaments on the surfaces of the membrane and of the stent,respectively, for selective local administration at the outside surfaceof the membrane facing vascular tissue. Examples of such medicaments areimmunosuppressive agents and anti-proliferative agents.

The stent is cooled e.g. by cold spray to a small diameter, say 2 mm.Then, a rectangular piece of the membrane material, preferably having atriangularly shaped outer end is wound so as to form a cylinder with aninner diameter sized so as to accommodate the contracted stent. Thetriangular end must be located on the surface of the membrane cylinder.If desired the cylinder shape can be fixated by a heat treatment of themembrane cylinder, before placing the crimped stent therein.

Then the handle 14 or manipulator piece is set up to receive thecylindrical assembly 2. Preferably the handle has the locking thread 20provided as a loop of a size larger than the diameter of thestent/membrane assembly, extending out from the aperture 22 located inthe support surface 18 for the stent/membrane assembly 2. Thecylindrical assembly is then placed on the support surface 18 within theloop 20.

The cold and contracted stent is positioned inside the lumen of themembrane cylinder, and the locking thread is pulled tight and the endsare secured. In this way the assembly will be maintained in a contractedstate, and can be subjected to packaging and sterilization procedures torender it ready to use.

In the case where the membrane is attached to the stent, the procedurefor assembling the device will differ slightly, but still comprisescooling of the stent. However, of course the membrane will be woundaround the stent, and the thread is tightened to keep the crimped state.

A vessel anastomosis may be performed as follows:

The two vessels to be joined are clamped by vascular clamps. Two lateraland one posterior stay sutures are applied, for the purpose of pullingthe ends together once the stent has been positioned appropriately.

The stent device is directed by means of the handle into one of the freevessel ends to half its length as set by the position of the handle.Then the other vessel end is passed over the opposite end of the stentassembly and the stay sutures are tied. Alternatively, the three staysutures may be tied to join the free ends of the two vessels beforeimplanting the stent device. When the stent is in place, an anteriorsuture is added. Now, the locking thread keeping the stent in a crimpedor contracted state is released, and the stent and the membrane willexpand simultaneously to the size of the vessel. The membrane providesimmediate and complete sealing, and also prevents a traumatising contactbetween the stent and the tissue. The thread is then pulled away and thehandle is easily removed from the anastomosis site. The vessel clampsare released and removed to re-establish blood circulation.

Thus, by means of the new device a tight and non-leaking connection oftwo blood vessels can be achieved by a very simple and quick procedure,which also ascertains an excellent sealing, which is leak-proofimmediately upon completion of the connection. A further advantageouseffect implied by rapid closure and restoration of blood flow, is thatthe ischemic time is substantially reduced, which in turn reduces thehazards involved when performing e.g. transplantation surgery.

In fact, the time for performing an anastomosis procedure on alaboratory animal (pig) was reduced from about 20-30 minutes using atraditional method down to 8-10 minutes employing the device and methodaccording to the invention.

It will be obvious that the invention as described may be varied in manyways. Such variations are not to be regarded as a deviation from thespirit and scope of the invention, and all such modifications as wouldbe obvious to one skilled in the art are intended to be included withinthe scope of the following claims.

1. A stent assembly (2), comprising a stent (4; 50) capable of assuminga contracted and an expanded configuration; a membrane (6; 58) of abiocompatible material impermeable to molecular transport across themembrane wrapped around said stent (4; 50) more than one full turn;characterized in that the membrane (6; 58) is made of a laminatedmaterial, comprising at least two layers with different thermalexpansion coefficients.
 2. The stent assembly as claimed in claim 1,wherein the end portions (8, 10) of the membrane are free relative toeach other and to the stent.
 3. The stent assembly as claimed in claim1, wherein the inner end portion (8) of the membrane is attached (9) tothe surface of the stent (4).
 4. The stent assembly as claimed in claim1, wherein the length of said membrane is such that the free endsthereof overlap also when said stent has been allowed to expand.
 5. Thestent as claimed in claim 1, wherein the surfaces of the stent and ofthe membrane are chemically or biologically modified.
 6. The stentassembly as claimed in claim 5, wherein the surface modificationcomprises immobilisation of heparin on the surface.
 7. The stentassembly as claimed in claim 5, wherein one or more of the surfaces onthe stent/membrane assembly is provided with a medicament for selectivelocal administration.
 8. The stent assembly as claimed in claim 7,wherein the medicaments are selected from immunosuppressive agents andanti-proliferative agents.
 9. The stent assembly as claimed in claim 1,wherein the outer end (10) of the membrane (6; 58) has a triangularshape (21; 62).
 10. The stent assembly as claimed in claim 10, whereinthe apex of the triangular end (21) is provided with a tab (32) ontowhich the thread (20) is attached such that a loop (34) is formed, saidloop extending out from the tab (32).
 11. The stent assembly as claimedin claim 1, wherein the stent is made of a shape memory material. 12.The stent assembly as claimed in claim 1, wherein the stent isexpandable from a contracted state by the force from a balloon providedon a catheter, whereby the stent is provided over said balloon.
 13. Thestent assembly as claimed in claim 1, wherein the end edges of the stent(50) are provided with a plurality of end stop means (52; 54)distributed around the periphery of the edges, for keeping the membrane(58) in position on the stent (50).
 14. The stent assembly as claimed inclaim 13, wherein the end stop means are formed from the same threadmaterial as the stent is made of, and constitute pins (52) pointingoutwardly essentially in the radial direction.
 15. The stent assembly asclaimed in claim 13, wherein the end stop means are formed as pellet(54) like structures.
 16. The stent assembly as claimed in claim 13,wherein the stent is made of a moldable material, and the end stop meansare formed during molding of the stent.
 17. A device for applying astent assembly, comprising a stent assembly (2) as claimed in claim 1 ina contracted state; a holding and gripping device (12) comprising ahandle portion (14) and a support structure (16, 18) for accommodatingsaid stent assembly a locking loop (20) that can be tightened around thestent assembly when it is positioned on said support structure, andreleased to allow said stent assembly to expand to a nominal size. 18.The device as claimed in claim 12, wherein said loop is formed by athread (20) extending through said handle, and protrudes out from saidsupport structure (16, 18) through an aperture (22) in the surface (18)of said support structure.
 19. The stent assembly as claimed in claim 2,wherein the length of said membrane is such that the free ends thereofoverlap also when said stent has been allowed to expand.
 20. The stentassembly as claimed in claim 3, wherein the length of said membrane issuch that the free ends thereof overlap also when said stent has beenallowed to expand.